1. Field of the Invention
This invention relates to a webbing for safety belts which serve to protect passengers in airplane accidents, vehicle accidents or like accidents.
2. Description of the Prior Art
Most of webbings for safety belts (hereinafter called simply "webbings" for the sake of brevity) are produced by needle looms these days. In each of such webbings, one of the selvedges is formed as a knitted selvedge and the other selvedges takes the form of a woven selvedge. As warp threads for these selvedges, the same threads as the warp threads of its main part, namely, threads having a relatively large denier size are used.
Due to the use of such large-diameter warp threads, the outer edge of the knitted selvedge tends to become uneven in the lateral direction to an extent as much as the diameter of at least one warp thread. In order to align outwardly-extending end loops of weft threads, it has been necessary to use an intertwining thread also known as a catch thread, of a relatively large denier size (250-500 denier). Another problem has however arisen from the use of such an intertwining thread that the end loops become unavoidably large.
Similarly, the woven selvedge presents an uneven outer edge because the turning points of weft threads vary in and out in the widthwise direction to an extent as much as the diameter of at least one warp thread. An additional problem is also involved that the weft threads project out from the warp threads due to the tensile force of the warp threads since the warp threads have a large denier size. (see, FIG. 6 in which a indicates warp threads and b denotes weft threads.).
For the above-mentioned reasons, the knitted selvedge and woven selvedge are thus accompanied by such inconvenience that while wearing the webbing, the manner of their contact to clothing is unpleasant and the feeling of their touch to the skin is hard especially in light dress.
As a solution for the above problems, there has been proposed a webbing in which at least one of the selvedges has a tubular shape and uses threads having a higher elongation than those of the main part of the webbing (see, U.S. Pat. No. 4,018,960 issued Apr. 19, 1977 to Johann Berger et al.).
In the above-proposed webbing, the warp threads of the selvedges are subjected to advance shrinkage or the denier size of the warp threads of the selvedges is either increased or reduced by employing twisted threads, so that the cutting of the main part and selvedges of the webbing can be effected simultaneously. Extreme difficulties are however encountered in the fabrication of the webbing, because not only the adjustment of elongation but also various other conditions such as weave and weaving conditions are correlated as a matter of fact.
Although the above-proposed webbing gives superior feeling to touch to conventional webbings, it is still accompanied by the following problems. The selvedges are in a tubular form and are each composed of two plies, one being a front ply and the other a back ply. When the webbing is bent by a guide, the front ply is stretched while the back ply in contact with a guide undergoes buckling. As a result, the warp threads of the back ply are loosened and are hence rendered susceptible to abrasion. In an earlier stage of its use, the back ply tends to develop fluffing due to cutting of its filaments, resulting in such problems that the appearance of the webbing is impaired, the user feels uneasy about the safety of the webbing and the smooth winding of the webbing into the associated retractor is disturbed.
With a view toward reducing the above problems, it has also been proposed to form the main part into a 4-up/4-down weave and to increase the density of weft threads in the selvedges. This proposal however involves such problems that the webbing becomes thicker, the associated retractor can take up the webbing over a shorter length thereof, and the fabrication cost of the webbing increases substantially.